Sediment Transport
Outline1. Incipient motion criteria for unisize and
mixed-size sediments2. Modes of sediment transport3. Bedload transport4. Suspended load5. Bedforms
Incipient Motion
(Middleton and Southard, 1984)
Forces Acting on Stationary Grain
(Middleton and Southard, 1984)
gDgDD
FF
G
D
0
3
20
Threshold of Motion
(Shields,1936; Julien, 1998)
(Miller et al., 1977)
gDc
c
Motion
Smooth Transitional Rough
045.0c
No Motion
Sample Calculation
What is c for D = 0.005 mm quartz-density particle?
Pa 6.3005.081.910002650045.0
gDcc
gDc
c
Entrainment of mixed-size sediment
Due to:1. Relative Protrusion2. Pivoting angle
Relative Protrusion
Pivoting Angle
Threshold of Motion for a Stationary Grain (Unisize or Graded Sediment)
Wiberg and Smith (1987), Bridge and Bennett (1992), + many others
, :Mixtures Dy
DLDG
DG
G
D
FFllll
FF
tan1sincostan
4.06.050045.0 ici DDg
Entrainment of mixed-size sediment
Sample Calculation
What is c for 0.001 and 0.010 m quartz-density particles in a mixture with D50 = 0.005 m?
Pa 8.401.0005.081.910002650045.0
m 0.01For Pa 9.1001.0005.081.910002650045.0
m 0.001For
4.06.0
4.06.0
4.06.050045.0 ici DDg Using Shields
for unisize sediment
0.7 Pa
7.3 Pa
Sediment Transport
(Leeder, 1999)
Modes of sediment transport
Criteria for Sediment Transport Modes
• Bedload:
• Suspended bed material:
• Washload: D 0.063 mm
c 0
suau *
(Bridge, 2003)
Modes of sediment transport
c 0
suau *
Washload:D 0.063 mm
Bedload Transport EquationsMeyer-Peter and Muller (1948)
Bagnold (1966)
323 18 gDggq cb
ccb uuaq
0**tan
Bedload traps (K. Bunte)
Helley-Smith sampler
Measuring bedload transport
Bedload Transport Observations
Gravel-bed stream (Cudden & Hoey, 2003)
Gravel-bed streams (Bunte et al., 2004)
fib Qfib
trap
HS
HS
Bedload Transport EquationsWilcock & Crowe (2003)
Reference threshold condition
Hiding function
Reference dimensionless shear stress for median size base don fraction of sand
Transport rate based on /ri
Bedload Transport EquationsMeyer-Peter and Muller (1948)
Bagnold (1966)
323 18 gDggq cb
ccb uuaq
0**tan
Barry et al. (2004)
Abrahams and Gao (2006;following Bagnold, 1966, 1973)
sss ddQ
qb
fqQAq
50502,,*
257 56.3*45.241.3
TTG
UGi
g
b
2
4.3
Barry et al. (2004)
Abrahams and Gao (2006)following Bagnold (1966, 1973)
Predicting bedload transport
(c) Ackers and White [1973] equation by di
(a) Meyer-Peter and Müller [1948] equation by d50ss
(b) Meyer-Peter and Müller equation by di
(d) Bagnold equation by dmss
(e) Bagnold equation by dmqb
(e) Bagnold equation by dmqb
(g) Parker et al. [1982] equation by di (Parker et al. hiding function)
(h) Parker et al. [1982] equation by di (Andrews [1983] hiding function)
(Barry et al., 2004)
Predicting bedload transport
Suspended Sediment
• Simple criterion for suspension: suau *
(van Rijn, 1993)
DH48 – Wading Sampler
DH59 – Hand line Sampler
D74 – Hand line Sampler
Others: Super-critical flumes, ISCO, OBS, Acoustics
Measuring suspended load transport
Suspended Sediment
• Sediment-diffusion balance (equilibrium):
downward settling + upward diffusion Total suspended load
• Rouse equation:
01 yCCCu ss
CC
d yy
ad aa
z
*u
uz s
h
adyuCqs
(van Rijn, 1993)
Suspended sediment profiles and Rouse equation
Z
Ripples
Dunes
Upper-stage plane beds Bedload sheet
Bedform Stability
Suspended Load Observations
Mobile river dunes with acoustic probe, Wren et al. (2007)
Stochastic simulation, Man (2007)
Mobile orbital ripples with acoustic probes, P. Thorne
Sediment Transport and Stream Restoration
• Deficient or excessive sediment transport based on design discharge will result in erosion or deposition, which can redirect flow and threaten infrastructure and ecologic indices
• Sediment transport prediction depends on grain size, gradation, and bed topography
• Uncertainty can be large• Excludes bank erosion and wash load• Use multiple relationships
Sediment Transport
Conclusions• Threshold conditions defined by Shields
criterion• Modes of sediment transport depend on
Shields criterion and grain size• Bedload and suspended load transport
treated separately• Load is modulated by bedforms